Headphones are classified broadly into three types, that is, an over-the-head type in which a headphone is held by a headband, an ear-hook type in which a headphone is held by being hooked on an ear, and an ear-insertion type in which a headphone is held by being directly inserted into an ear.
Among these types, the over-the-head type headphone is popular for the following reasons: high sound quality is obtained because of the use of a headphone case large enough to cover the entire ear; and even long-time fitting puts a low load on the ear.
According to a first conventional example of the over-the-head type headphone, as illustrated in FIG. 3A, an over-the-head type headphone 1A includes a headphone band (not illustrated) that is fitted along a head H in an arch-like manner, and a headphone case 2 is suspended at each end of the headband.
A headphone unit 3 is housed in the headphone case 2 while being supported by a baffle plate 4. A doughnut-shaped ear pad 5 to be placed around an ear E at the time of fitting of the headphone is integrally attached to a sound emitting surface of the headphone case 2. In many cases, a dynamic electro-acoustic transducer is adopted for the headphone unit 3.
In the headphone case 2, a back air chamber C2 having a predetermined volume is formed by the baffle plate 4 on a back side of the headphone unit 3, and a substantially hermetically sealed front air chamber C1 is formed by the ear pad 5 also on a front side of the headphone unit 3 at the time of fitting of the headphone.
In the headphone 1A of this type, the following is a common practice to attenuate in-head localization of sound images and improve sound quality and frequency response. That is, an acoustic channel 6 is provided to part of the baffle plate 4, an acoustic resistance material 6a such as non-woven fabric is attached to the acoustic channel 6, and the front air chamber C1 and the back air chamber C2 are acoustically connected to each other.
Meanwhile, in order to obtain a headphone having excellent sound quality, it is preferable to increase the diameter of the headphone unit 3 as much as possible. If a large-diameter headphone unit 3 is mounted as it is on the headphone 1A having the configuration illustrated in FIG. 3A, however, the size of the headphone case 2 increases accordingly, and the portability thereof is impaired.
For the purpose of avoiding the portability thereof from being impaired, in order to mount the large-diameter headphone unit 3 without such an increase in size of the headphone case 2, the acoustic channel 6 provided to the baffle plate 4 needs to be placed in the vicinity of the lower side of the ear pad 5 (the side of the ear pad 5 opposed to the baffle plate 4).
If such a design is adopted, however, the acoustic channel 6 may be closed by the ear pad 5, depending on a lateral pressure applied when the headphone is fitted to the head H.
In view of the above, in a headphone 1B (second conventional example) according to an invention described in Japanese Patent Application Publication No. 2013-42382, which is illustrated in FIG. 3B, in order to make a larger-diameter headphone unit 3 mountable, a communication groove 4a is formed in the baffle plate 4 in the case where the acoustic channel 6 provided to the baffle plate 4 is placed on the lower side of the ear pad 5. The communication groove 4a is communicated with the acoustic channel 6, and is deep enough not to be closed by the ear pad 5.
According to Japanese Patent Application Publication No. 2013-42382, it is possible to provide a headphone that has a small size but includes the large-diameter headphone unit 3, in which the acoustic channel 6 is hardly closed by the ear pad 5. However, because the headphone still includes the baffle plate 4, there is a limit to an increase in diameter of the headphone unit 3. Moreover, because it is necessary to form, in the baffle plate 4, the communication groove 4a that is deep enough not to be closed by the ear pad 5, production costs become higher accordingly.
Under the circumstances, the present invention has an object to provide a dynamic headphone that has a small size but makes a larger-diameter headphone unit mountable thereon, the dynamic headphone including a highly reliable acoustic channel that acoustically connects a front air chamber and a back air chamber to each other.